Power rectifiers find wide applications in electronic device and power system etc. The design issues of power rectifiers are low turn-on voltage, high switching speed, and high breakdown voltage. The property of high breakdown voltage is especially important in the application of power supply. A power rectifier with a higher breakdown can more probably offer a surge and provide a safer device.
FIG. 1 shows the structure of a conventional power rectifier. The device is formed by epitaxially growing a lightly-doped n type impurity layer 12 over a heavily doped n.sup.+ type substrate 10, and then diffusing a heavily-doped p.sup.+ impurity layer 14 over the layer 12. The resultant structure is provided with anode electrode and cathode electrode above p.sup.+ impurity layer 14 and below n.sup.+ type substrate 10, respectively, to form a power rectifier with a p-n junction.
The power rectifier described above should prevent the prematurity of avalanche breakdown to ensure a high breakdown voltage. More specially, the occurrence of a crowded electric field within the depletion region of the power rectifier should be prevented. For examples, the conventional approaches to enhance the breakdown voltage are to provide floating field rings or field plates within the depletion region. The electrical field distribution will be prevented from being crowded by the provision of the field rings or field plates.
However, in the structure shown by FIG. 1, the n type impurity layer 12 is formed on the heavily doped n.sup.+ type substrate 10 by epitaxy. Micro defects such as dislocations are probably occurred and propagated from the interface between the substrate 10 and the impurity layer 12. The prior art results in a dislocation occurring between the epitaxy layer and the substrate. The dislocation generally will not cause an avalanche breakdown when a low voltage (e.g. &lt;450 V) is applied to the power rectifier. However, the dislocation originally occurred between the epitaxy layer and the substrate will influence the p-n junction atop as a higher voltage (e.g. &gt;450 V) is applied. The dislocation caused by the high applied-voltage will induce a snap-down problem which decreases the breakdown voltage.